1. Field of the Invention
The present invention relates to a liquid discharge head that includes a heat generating element generating thermal energy for discharging a liquid.
2. Description of the Related Art
A recording apparatus such as a printer, a copying machine, and a facsimile records an image formed from a dot pattern on a recording material such as a thin paper or a plastic sheet on the basis of image information. The dot pattern is formed by, for example, a liquid such as ink. The recording apparatus may be largely classified into an ink jet type, a wire dot type, a thermal type, a laser beam type, and the like in accordance with a recording type. Among these types, the ink jet type recording apparatus performs a recording operation by discharging liquid droplets from a discharge port of a liquid discharge head so as to be adhered to a recording material.
In recent years, a number of recording apparatuses have been used, and in these recording apparatuses, there have been an increasing demand in which the recording operation needs to be silently performed at a high speed with high resolution and high image quality. As one of the recording apparatuses satisfying the demand, the ink jet type recording apparatus may be exemplified. In the ink jet type recording apparatus, a recording operation is performed by discharging a liquid from a liquid discharge head. For this reason, in order to satisfy the aforementioned demand, the liquid needs to be stably discharged while ensuring a stable discharge amount of the liquid. Stability in liquid discharge is largely influenced by the temperature of the liquid discharge head.
Particularly, in the recording apparatus configured to form a bubble in solid ink or liquid ink by using thermal energy and to discharge the ink as an ink droplet, the discharge characteristics greatly change due to the temperature of the liquid discharge head. Further, since there is a restriction in the time (refill frequency) until a liquid chamber (bubbling chamber) provided in the liquid discharge head is filled with a liquid after the liquid is discharged to the outside, increases in recording speed are restricted. However, in recent years, a liquid discharge head capable of performing a rapid printing operation has been developed, whereby the printing operation may be performed much faster than that of the related art.
However, when the recording operation is rapidly performed, the amount of accumulated heat increases, so that the liquid may not be stably discharged to the outside. Particularly, a problem arises in that the amount of liquid to be discharged becomes irregular due to a rising temperature. In order to solve the irregular discharge amount of the liquid, Japanese Patent Application Laid-Open No. 2005-280068 discloses a structure in which the temperature of a head is detected, and the discharge ratio between a large dot (liquid droplet) and a small dot (liquid droplet) changes on the basis of the detection result. Further, Japanese Patent Application Laid-Open No. H08-156258 discloses a structure in which the number of liquid droplets to be discharged is counted, and the application time of a voltage applied to an electric thermal conversion element as a heat generating element is controlled on the basis of the counted number.
In the recording heads disclosed in Japanese Patent Application Laid-Open No. 2005-280068 and Japanese Patent Application Laid-Open No. H08-156258, when there is a difference in temperature distribution for every discharge port array, a problem arises in that a temperature control method needs to be changed for each discharge port array so that the liquid discharge performance for each discharge port array attains a predetermined liquid discharge performance.
Particularly, in recent years, the recording operation has been conducted at the high speed with the high duty, the low pass, and the elongated nozzle. For this reason, a temperature of a substrate (head substrate) constituting a liquid discharge head may partly increase due to the recording operation. As a result, even in the recording operation by one scanning operation, a difference in discharge amounts occurs for each discharge port array, so that the concentration of a recorded image becomes remarkably irregular.
Further, the number of interconnections decreases and the size of a circuit decreases in accordance with the advanced technology, which realizes a decrease in the size of a substrate and enables a design in which more substrates may be manufactured from one silicon wafer. As a result, as shown in FIG. 24, a volume of a substrate portion 802 in the periphery of each heat generating element 801 provided in a discharge port array 800 (nozzle array) may be different. Specifically, a head substrate 803 is divided into plural substrate portions 802 by common liquid chambers 804 supplying a liquid to a nozzle and extending in a shape of plural lines, and the volume of each substrate portion 802 provided with each discharge port array 800 is different.
In the substrate portion 802 (a portion located at the end portion of the substrate in FIG. 24) having a small volume, a thermal diffusion portion radiating heat to the nozzle array is small. For this reason, a problem arises in that a temperature remarkably increases in the vicinity of the discharge port array 800 formed in the substrate portion 802 having a small volume rather than the vicinity of the discharge port array 800 formed in the substrate portion 802 having a large volume. Particularly, the substrate portion 802 located at the end portion of the substrate contacts a sealing material or atmosphere, and the sealing material or the atmosphere has thermal conductivity and specific heat smaller than that of the liquid inside the common liquid chamber. Accordingly, the heat radiation performance of the substrate portion 802 located at the end portion of the substrate becomes smaller than that of the substrate portion 802 interposed between the common liquid chambers 804.
In a system in which a variation in thermodynamic state may be disregarded at the time of the input or output of thermal energy, thermal capacity is dependent on the amount of material and the specific heat or thermal conductivity thereof. Further, the interval between the adjacent heat generating elements is becoming narrower due to increasing density such as in a nozzle of 1200 dpi and the like. For this reason, when the heat generating element continuously radiates heat, rising temperature during one scanning becomes more apparent.
As described above, due to differences in thermal capacity, thermal conductivity, specific heat, and the like around each heat generating element, a large difference in temperature distribution of the substrate portion around each nozzle array occurs. When the temperature distribution is largely different for each nozzle array, it is necessary to perform particular control in accordance with the temperature distribution for each nozzle array in order to realize a recording operation without irregularity. Further, when each nozzle array needs to be controlled, it is necessary to further install a temperature sensor in order to improve the measurement precision of the temperature distribution. Further, when the temperature is controlled for each nozzle array, there are problems in that the control system becomes complex and the number of interconnections increases. Further, there are problems in that a difference in temperature distribution occurs even in a recording operation of a single scan and irregularity in recording operation occurs.